Solubility solvent evacuation

Crystallisation by Evaporation.—This method is employed when the substance is so easily soluble in all solvents (hot and cold) that it will only separate after partial evaporation. The solvent is allowed to evaporate spontaneously in the air or in a desiccator if in the latter the evaporation is greatly hastened by using a suitable absorbent as well as evacuating the desiccator. The type of vessel employed depends on the volatility of the solvent obviously the conical flask already recommended for crystallisation by cooling is not suitable for spontaneous evaporation, while a beaker or shallow crystallising dish is. When the latter type of vessel is used, crusts often form on the sides above the surface of the liquid. Such crusts seldom consist of pure substance, and they should be carefully removed with a spatula before attempting to filter off the crystals. 

Crystallization is performed in Schlenk apparatus under an inert atmosphere using familiar techniques partial removal of solvent by evacuation, addition of a solvent in which the solute has reduced solubility, and cooling. These methods are often combined for example, a second, poorer solvent, chosen so that its vapor pressure is less than that of the more powerful, original solvent, may be added. Partial evacuation will then concentrate the solution and change the solvent composition to favor crystallization. The complication which air-sensitive... 

The vapor pressure of /i-[(CH3)2N]B2H5 obeys the relationship log P = 2158.56/T + 1.75 log T - 0.008061T + 7.518831 (101 torr at 0°). The gas-phase infrared spectrum has been reported in detail.5 The compound is a useful intermediate in the synthesis of other boron nitrogen compounds, including those containing NBNB6 and PBNB7 chains, and Na[(CH3)2-N(BH3)2].8 The compound can be stored at 25° for months in sealed, evacuated Pyrex tubes. It is soluble in ethers and aromatic hydrocarbons, but is attacked by protic solvents. 

Physical Data mp 160-161 °C (sealed evacuated capillary). Solubility sparingly sol common organic solvents, i.e. THF, CCI4, CH2CI2, CHCI3. The suspended material is active. 

In each experiment, the substrate (40 g), catalyst (0.2 g, dry basis), aqueous ammonia (24 ml), and methanol were charged to the RCl reactor. The mixture was stirred at 16(X) rpm under Nj at the reaction temperature to dissolve the substrate. The substrate was fully soluble in the solvent under these conditions. To initiate the hydrogenation reaction, Nj was rapidly evacuated by a mechanical pump, followed by pressurizing the reactor to 40 psig H,. This evacuation-and-fiU procedure was conducted with the agitation speed at 1600 rpm to ensure rapid and complete degassing of the inert gas from, and rapid incorporation of Hj into the liquid phase. The process usually took < 10 s. 

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